36 BRITISH TUNICATA. 



The test of Mo~l<ju~l<i is thin, tough, and membranous, 

 and is usually, if not always, covered with hair-like 

 fibrils sometimes a little branched, by which it attaches 

 a coating of sand or shelly particles. It is, with one 

 or tw r o exceptions, only very slightly adherent to the 

 mantle, excepting at the respiratory tubes, Avhere the 

 attachment is firm. The mantle is very delicate and 



t 



hyaline, and has a few muscular fibres radiating from 

 the base of the respiratory tubes. These pass up the 

 tubes, which are also supplied with circular fibres. 



The mantle is richly supplied with a plexus of 

 minute channels or vessels, and so is the intestinal 

 tube, as well as the membranous branchial rod. The 

 branchial suspenders are numerous and well developed, 

 and, supplying the test, there are two vascular trunks 

 which issue from the left side of the mantle or inner 

 tunic near to the dorsal end of the heart. It may 

 therefore be infereritially determined that the circula- 

 tory organs are of the same character, and are as 

 complete, as they are in Axci<Ji. The heart is very 

 distinct. It lies in the left-hand wall of the mantle, 

 immediately below, and in contact with, the hard cylin- 

 drical body about which little has been determined. 



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The branchial sac is short and wide, extending, 

 however, the full length of the pallial chamber, within 

 which it is placed somewhat diagonally. There are 

 usually six or seven wide, longitudinal folds on each 

 side. In M. coi//i>l<tu<tf<t there are six on the right side 

 and seven on the left. These folds extend the entire 

 leno'th of the sac ; but, as the ventral border is much 



O 



shorter than the dorsal, those next the ventral line are 

 not nearly so long as the dorsal ones, and they are all 

 considerably arched, the concavity being turned for- 

 wards. The deep grooves, formed in the outer surface 

 of the branchial sac by the inversion of the internal 

 folds, are each turned into a series of pouches in con- 

 sequence of the formation of septa at the points where 

 the primary or transverse vessels cross them. The 

 whole of the branchial surface between the transverse 



